The present invention relates to a digitally operating device for regulating the amplitude of an electrocardiographic signal.
In a number of biomedical instruments, electrocardiographic signals are used as synchronization signals for electrotherapeutic measures or for the detection or display of signals. The operating conditions fluctuate greatly during operation due, for example, to the inherently unstable nature of the contact provided for derivation of such signals between the transducing electrodes and the patient, and the resulting fluctuating signal amplitudes require a constant adaptation of the amplification in the transmission channel because, on the one hand, if transmission should occur with too much gain, the noise signals contained in the derived signal may be confused with pulses of the QRS complex and, on the other hand, too little amplification will cause the QRS-complexes to be submerged in the noise signals or the occurring amplitude peaks will no longer suffice to actuate the synchronization processes.
The detection of EKG signals is not only made more difficult by their low and changing frequency, but is additionally adversely influenced by occurring interference signals, or artifacts, whose amplitude often exceeds that of the useful signals by a factor of more than one hundred.
It is already known to attempt to deal with these difficulties by effecting an amplitude regulation manually by means of separate operating elements which required additional personnel.
German Offenlegungsschrift [Laid-open Application] No. 2,032,102 discloses a digitally operating circuit arrangement for automatically presetting the gain of an amplifier for electrocardiographic signals in which the input signal is rectified and fed to an integration circuit, one capacitor being charged to the peak value of the positive input signal and one capacitor being charged to the peak value of the negative input signal and being discharged if a lower signal or no signal appears. The thus stored actual value is compared with a desired value and a reciprocal setting member is caused to follow changes in the comparison result in that a counter which receives its clock pulses from a separate oscillator counts upward or downward.
Due to the existing stability conditions, such control devices require integration time constants of the order of magnitude of more than 30 seconds. Such integration constants require either the use of very large capacitances or very low currents, so that difficulties arise in many situations where it is important, for example, that only a small amount of space be occupied by the control devices or that they have a long term stability, such as for use within implantable cardiac pacemakers. Moreover, in such a device the above mentioned high amplitude interference signals, as they are produced, for example, from short-term changes in the contact resistance of the electrodes, generate a regulating effect which considerably reduces the sensitivity of the system and, due to the long regulating time constant, the output amplitude of the QRS complexes returns to the amplitude required, for example, for synchronization purposes, only after a longer period of time.
A number of digitally operating amplitude regulating devices for general application or various specific applications have been described in a multitude of publications. However, such devices will operate at their best only if they are specially adapted to the signals to be processed.
For example, German Offenlegungsschrift No. 2,158,985 discloses a device for regulating the degree of amplification in an alternating voltage amplifier in which, for use in the receiver art, threshold switches are provided which have monostable retriggerable switching states, are set into the astable state when the threshold is exceeded, and maintain this state for a period of time which is slightly longer than the period duration of the fed-in input voltage. The set threshold switch then enables a gating circuit which activates a counter to cause the transmission factor to be reduced in a plurality of steps.
Although in this arrangement no integrating members are used for the input signal, the monostable threshold switch at least contains a timing member including capacitances. Moreover, the regulation operates in a direction toward reducing the amplitude of the output signal. Because of the longer time intervals between successive QRS complexes, the operation performed by this device is not suitable for processing electrocardiographic signals.